aMolecular Sciences Institute, School of Chemistry, University of the Witwatersrand, PO Wits 2050, Johannesburg, South Africa*Correspondence e-mail: Manuel.Fernandes@wits.ac.za

(Received 2 December 2011;accepted 7 December 2011;online 14 December 2011)

In the title compound, C13H9NO8·CH3OH, the main mol­ecule possesses three carb­oxy­lic acid groups, which are asymmetrically distributed around the mol­ecule core. This results in hydrogen-bonding motifs ranging from a chain to various rings. The combination of the chain motif together with a carb­oxy­lic dimer R22(8) ring motif creates a ribbon of mol­ecules propagating along the c-axis direction. A second ribbon results from the combination of the chain motif together with a methanol solvent mol­ecule and carboxyl-containing R44(12) ring motif. These two ribbons combine alternately, forming a hydrogen-bonded layer of mol­ecules parallel to (20).

Charge transfer complexes composed of pyromellitic anhydride have been extensively studied for their electronic properties. These molecules have also been modified into various imides or polyimides by reaction with suitable amines for use as host guest materials (Barooah et al., 2006), gas separation materials (Kim et al., 2002; O'Brien et al., 1988), and semiconductor materials (Dingemans et al., 2004; Zheng et al., 2008). Such products are usually symmetric with very few asymmetric examples of these products having been reported. A search of the REAXYS database (Elsevier, 2011) of reactions involving pyromellitic anhydride as starting material resulted in 1083 hits. Of these, very few report asymmetric products and only five reported reactions result in one side of the molecule being converted to an imide while the other side is opened resulting in a di-acid. A search of asymmetric pyromellitic anhydride derived molecules in the Cambridge Structural Database (CSD; Version 5.32 release; Allen, 2002) indicates that only one asymmetrically substituted molecule (a diimide) has been reported (Zhu et al., 2010). No structure involving a pyromellitic molecule which has been converted to an imide on one side, and had the other side ring opened to form a di-acid has been reported. Due to asymmetry in carboxylic acid substitution, such a molecule should result in an interesting and unusual hydrogen bonded network.

The title molecule (Fig. 1) has three carboxylic acid groups capable of H-bonding distributed unevenly as two ortho to each other on one side of the molecule, and another as a propionic acid extending out from the imide group on the other side of the molecule. This difference in carboxylic acid location results in different hydrogen bond patterns on the opposite sides of the molecule.

The crystal structure is composed of hydrogen bonded layers of molecules which are stacked along the [-2 1 0] direction. Each layer is held together by several hydrogen bonds (Fig. 2). On the imide side, the single propionic acid hydrogen bonds to another on a neighbouring molecule (related by an inversion center) to form a carboxylic acid dimer which can be described by the graph set R22(8) (Etter et al., 1990; Bernstein et al., 1995). On the other side of the molecule, one of the carboxylic groups hydrogen bonds to a carbonyl group of a neighbouring molecule (related by translation along c) to form a chain which can be described by the graph set C(9). The combination of the C(9) chain and the R22(8) dimers results in a ring of hydrogen bonded molecules described by the graph set R44(40). This upon cell translation produces a ribbon of molecules down the c-axis. A second different ribbon exists on the edges of the one just described. This is formed by the remaining carboxylic acid group which together with the methanol molecule forms a centrosymmetric hydrogen bonded ring of molecules [graph set R44(12)] to link the previously mentioned ribbons together. The combination of the two alternating ribbons results in a hydrogen bonded layer of molecules parallel to (2 -1 0).

The title compound was accidently synthesized in a crude yield of 35% by reaction of pyromellitic anhydride with beta-alanine in a 1:1 molar ratio by refluxing in DMF containing water as an impurity. The product from the reaction was recrystallized for analysis by X-ray diffraction from methanol by means of slow evaporation at room temperature resulting in colorless needle-like crystals.

All H atoms were positioned geometrically, and allowed to ride on their parent atoms, with Atom—H bond lengths of 0.95 Å (CH), 0.99 Å (CH2), 0.98 Å (CH3), or 0.84 Å (OH). Isotropic displacement parameters for these atoms were set to 1.2 times Ueq of the parent atom for CH and CH2, and 1.5 times Ueq of the parent atom for CH3 and OH.

This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.